Chondrocytes derived from chick embryo sternae are differentiated cells typical of cartilage which exhibit several unique biochemical characteristics when grown in vitro. They synthesize a cartilage-specific proteoglycan and a unique collagen (type II) which is readily distinguished from collagens synthesized by other cell types. Previous studies have shown that such specific chondrogenic expression is inhibited when chondrocytes are exposed to 5-bromodeoxyuridine. In particular, such dedifferentiated chondrocytes no longer synthesize type II collagen; type I collagen, typical of chick embryo fibroblasts, is produced instead. As an approach to understanding normal and abnormal cartilage development, we are examining the molecular basis for the loss of phenotypic traits of chondrocytes after exposure to 5-bromodeoxyuridine. Using recombinant DNA technology, we are (a) measuring the levels of type I vs. type II collagen RNA in different cell populations, (b) examining the genomic structure of such cells, and (c) determining if 5-bromodeoxyuridine alters the transcriptional state of the different collagen genes.